It is proposed to determine to better than 1.7 Angstroms resolution the crystal structure of the profilin:beta-actin complex using X-ray crystallography, to use time-resolved Laue diffraction to characterize the extensive structural transitions inducible in profilin:beta-actin crystals, to obtain a model of the filamentous form of beta-actin, and to determine the structures of profilin:gamma.actin, vitamin D binding protein, and Birch pollen profilin. The highest possible resolution structure of actin is essential for understanding the role of actin in chemo-mechanical transduction and, in particular, the process of muscle contraction. In nonmuscle cells, very basic processes such as cytokinesis, membrane ruffling, stress fiber formation, microvillar vesiculation, amoeboid movements, and nerve growth cone motility depend upon control over actin filament formation and organization into higher order structures. Profilin plays a central role in the regulation not only of actin polymerization, but also of enzymes that generate second messengers in response to ligand-receptor interactions at the cell surface. It thus serves as a key link between signal transduction complexes formed in response to cellular activation and the actin microfilament system. The structure of profilin in complex with actin will be important in unraveling the cytoplasmic events occurring when a cell, transformed by retroviruses or growth factors, undergoes dramatic changes in cytoplasmic organization and dynamics. The structure of profilin:actin will be of importance in understanding cancer because of its role in controlling the appearance of the transformed phenotype. It is related to actions of the immune system in many different ways, including patching and capping of lymphocytes and macrophage motility. The structure of pollen profilin will contribute to a deeper understanding of the allergic reaction.